Method and an arrangement for controlling position and/or force of an elongated rolling device

ABSTRACT

In paper and board machines the position of a rolling device relative to another rolling device and/or the force exerted by the rolling device on the other rolling device or any variable acting on these is measured, and the value of the measured variable is compared with the set value of said variable in order to obtain the difference value of the variable, and the position of the rolling device and/or the force it exerts on the other rolling device is controlled on the basis of the difference value. The fluid pressure of the hydraulic means ( 5 ) and/or the flow velocity of the fluid to the hydraulic means is changed in order to alter the difference value of the variable by opening and/or closing at least one digital valve in a digital valve pack ( 7 ) functionally connected to the hydraulic means (S).

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a U.S. national stage application of InternationalApp. No. PCT/FI2003/000860, filed Nov. 13, 2003, the disclosure of whichis incorporated by reference herein. This application claims priority onFinnish App. No. 20022030, filed Nov. 14, 2002.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH AND DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

The invention relates to a method and an arrangement for controlling theposition and/or force of an elongated rolling device in the roll nipbetween two elongated rolling devices in paper and board machines.

The nip pressure in a roll nip between two rolls and the opening andclosing of the roll nip are adjusted with hydraulic means connected tosaid rolls, such as hydraulic cylinders. For nip pressure control,suitable measuring means are first used for measuring the force/pressuregenerated by the roll in the roll nip, the control logic of the controlsystem converts an analog measurement signal into a digital signal andtransmits a control signal in digital form to the control valve incharge of changing the nip pressure. The digital control signal isconverted into analog form by the control valve, and then the controlvalve controls the fluid flow entering and leaving the hydraulic means.Such a manner of controlling nip pressure has noticeable shortcomings,of which the major ones relate to disappearing data content as an analogmeasurement signal is converted into digital form and a digital controlsignal is subsequently converted into a control signal.

There are frequently also problems caused by the fact that the samerelatively large-sized control valve, such as a proportional valve, isused for controlling both the force exerted by the roll on the backingroll in the roll nip between the rolls and also the roll positionrelative to the backing roll. This problem is particularly tangible inreelers, because, as the fiber web is reeled around the reel core, thereel core needs to be continuously shifted away from the reelingcylinder. However, meanwhile it is necessary to maintain the nippressure between the reel core and the reel cylinder on a determinedlevel. The shift of the location of the reel core requires relativelylarge movements of the piston of the hydraulic means and also changes ofthe fluid pressure prevailing in the compression cylinder, whereaschanges of the nip pressure can be achieved with considerably smallerpiston movements and changes of the fluid pressure in the compressioncylinder, entraining a tendency to cause control fluctuation andvibrations in the roll/rolls. In practice, due to the great mass of thecontrol valve and the consequently slow changes of the flow volume inthe hydraulic means, it is often difficult or even impossible toactively attenuate roll vibrations caused by control fluctuation bymeans of control engineering means.

Controlling hydraulic means by current control valves such as servovalves and proportional valves is awkward and inaccurate, because therequired valves are bulky and slow, and thus have poor controlresolution. In addition, the control valves themselves might causecontrol fluctuation and vibrations in the rolling devices by their ownoperation.

The purpose of the invention is to eliminate the prior artinconveniences. Thus, the first purpose of the invention is to achieve asystem for controlling the location and the force of the roll, allowingthe same hydraulic means to accurately control both the location of theroll relative to the backing roll in the roll nip and also the nippressure (=force) generated by the roll in the roll nip, substantiallywithout control fluctuation. A second purpose of the invention is toachieve an active manner of control enabling efficient attenuation ofroll vibrations.

SUMMARY OF THE INVENTION

The invention relates to a method and to an arrangement for adjustingthe location and/or force of an elongated rolling device in the roll nipbetween two elongated rolling devices.

The invention is based on the feature of controlling the nip pressure ofa roll nip and the opening and closing of the roll nip with a hydraulicmeans, the volume flow arriving to the hydraulic means being at leastpartly controlled by a digital valve pack. The control signals utilizedby the digital valve pack and transmitted by the control system are bothin digital form, achieving the notable benefit over analog valves thatcontrol information does not require conversion from digital to analogform, so that no information will be lost while a digital control signalfrom the control system is converted into an analog control signal.

Use of the digital valve pack as switch means, allows very accuratecontrol of the volume flow reaching the hydraulic means; thus, forinstance, replacement of a large proportional valve with a digital valvepack containing 12 on/off digital valves provides a control resolutionof 4096 different volume flows. What is more, on/off digital valves havemarkedly fast operation, so that the same digital valve pack allowscontrol of the same hydraulic means both during shifts of the rolllocation, requiring large volume flow changes, while closing and openingof the roll nip, and also during changes of the nip pressure requiringrelatively small volume flow changes.

In this patent application, at least one of the rolling devices in theroll nip between two elongated rolling devices is a roll used in paperand board machines, such as a calendaring roll or a reeler roll. Theother of the rolling devices can then be a roll or an elongatedroll-like array, such as a doctor blade, or the blade of a coatingapplicator used in fiber web coating, without being confined to these,however.

A digital valve stands for a valve having N^((NUMBER OF VALVES)) states;and between two successive states, the valve is driven directly from thefirst state to the second state.

The valve preferably has two states; it is either completely open orcompletely closed. When the valve is open, it is permeated by the entirevolume flow rate of fluid allowed by this particular valve, and when thevalve is closed, it is not permeated by fluid at all. In thisapplication, a digital valve having two states is also referred to as anon/off valve and an on/off digital valve. A digital valve may have morethan two states, and then the valve is driven stepwise from one state toanother. The digital valve preferably has three positions; the valvetransmits fluid flow into a first and a second direction, or then thevalve does not transmit fluid. A digital valve pack including suchdigital valves having three states then has N³ states, in which N is thenumber of valves in the digital valve pack.

In the method of the invention for adjusting the location and/or forceof an elongated rolling device in the roll nip between two elongatedrolling devices in paper and board machines, the location of the rollingdevice relative to the other rolling device and/or the force exerted bythe rolling device on another rolling device or any variable acting onthese are measured, and the measured variable value is compared with theset value of said variable to obtain the difference value of thevariable. The difference value is used for adjusting the location of therolling device relative to the other rolling device and/or the forceexerted by the rolling device on the other rolling device. The fluidpressure of the hydraulic means and/or the flow velocity of the liquidto the hydraulic means is altered in order to change the differencevalue by opening and/or closing at least one digital valve in a digitalvalve pack functionally connected to the hydraulic means.

The arrangement, in turn, includes a measurement means for measuring thelocation of the rolling device and/or the force it exerts on the otherrolling device, or any variable acting on these, and for transmitting ameasurement signal to the control system. The arrangement furthercomprises a hydraulic means, by means of which the location of therolling device is shifted relative to the other rolling device and/orthe force exerted by the rolling device on the other rolling device inthe roll nip is changed, a switch means for adjusting the volume flow ofthe hydraulic means, a control system for receiving a measurement signaland for comparing the information in the measurement signal with the setvalue of the variable in order to provide a control signal and totransmit it to the switch means. The switch means has receive means forreceiving and processing a control signal and also at least one digitalvalve pack, which comprises digital valves, preferably on/off digitalvalves, which can be switched on and off on the basis of a controlsignal, so that the fluid pressure of the hydraulic means and/or theflow velocity of the liquid to the hydraulic means change.

In a preferred embodiment of the invention, the fluid pressure of thehydraulic means and/or the flow velocity of the fluid to the hydraulicmeans is changed on the basis of a digital control signal from thecontrol system by means of the digital valve pack, without convertingthe control signal into analog form in the meantime. Then themeasurement means generates an analog measurement signal, on the basisof which the control system transmits a digital control signal to thedigital valve pack that changes the flow rate and/or the fluid pressureof the hydraulic means.

In the invention, the control signals received and used by the digitalvalve pack are digital and the control signals from the control systemto the digital valve pack are already in digital from, so that thecontrol signal does not require conversion from digital form into analogform, as would be the case if the liquid flow of the hydraulic meanswere adjusted with an analog control valve. This achieves the markedadvantage over analog valves, that control information cannot be lostbetween the control system and the switch means (digital valve pack).

In another preferred embodiment of the invention, the location of therolling device in the roll nip and the force it exerts on anotherrolling device in the roll nip are adjusted by the same hydraulic meansand the amount and velocity of said volume flow of the hydraulic meansare changed by means of one or more digital valve packs.

In a further preferred embodiment of the invention, the measurementmeans performs measuring of the amplitude and frequency of the rollvibration and the control system determines the counter vibration forthis rolling device vibration (difference value), on the basis of whichselected digital valves in the digital valve pack are opened and closed.The counter vibration should be such that the amplitude of the measuredroll vibration decreases towards its set value.

In the last mentioned embodiment of the invention, a digital valve packallows for active vibration attenuation of the roll in a roll nip,unlike analog control valves. Using digital valves, the volume flow ofthe hydraulic means can be rapidly and accurately increased anddecreased with good volume flow resolution, so that even minorvibrations in the roll nip can be attenuated. This offers the furtherpotential feature of using the digital valve pack alongside aconventional analog control valve, such as a proportional valve; thecontrol valve serves to open/close a roll nip between the rolling deviceand possibly also to control the nip pressure between two rolling devicein the roll nip. The vibration of the rolling device in the roll nip isattenuated with active control operations by using digital valvesalongside the analog valves mentioned above for controlling the volumeflow to and from the hydraulic means.

The invention is described below in further detail with reference to theaccompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a roll nip between two rolls viewed from the end of theroll pair, and also the arrangement used for controlling the nippressure in the roll nip.

FIG. 2 also shows a roll nip between two rolls viewed from the end ofthe roll pair, and the arrangement used for controlling the opening andthe closing of the roll nip.

FIGS. 3A and 3E show a roll nip between two rolls viewed from the end ofthe roll pair. The figures illustrate the apparatus used for attenuatingvibrations of the roll nip. FIGS. 3B to 3D show the attenuation ofvibrations generated in the apparatuses by using the arrangement of theinvention.

FIG. 4 is a schematic view of the roll nip between the reel cylinder ofa reeler and the reel core, viewed from the end of the roll pair formedby the reel cylinder and reel core, and also the arrangement used forcontrolling the location of the reel core of the reeler and the nipforce.

FIGS. 5A and 5B shows a roll nip viewed from the end of the pair ofrolls in an apparatus used for fiber web coating, and the arrangementused for opening and closing the roll nip and for controlling the nippressure.

FIG. 6A is a schematic lateral view of a multi-zone roll and of thecontrol arrangement used for pressurizing its different zones. FIG. 6Bshows an arrangement for controlling a multinip calender using themulti-zone roll of FIG. 6A as the lowermost and the uppermost roll.

FIG. 7A is a block view of the arrangement of the invention and FIG. 7Bis a block view of the method of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following examination starts with the main features of the designsand functions of the illustrated apparatuses and also the object to beillustrated by each figure.

FIG. 1 shows a simple roll nip N between the rolls of a pair 2 of tworolls, the nip pressure being controlled with the control arrangement 1of the invention. The control arrangement comprises a hydraulic actuator5, a measurement means 4, a digital valve pack 7 and a control system 3.

FIG. 2 also shows a simple roll nip N between the rolls of a roll pair2, the roll nip being opened and closed with the control arrangement 1of the invention. The control arrangement includes a hydraulic actuator5, the pressure of the hydraulic fluid prevailing on different sides ofthe cylinder relative to the piston head being controlled with twoseparate digital valve packs 7; 7 a, 7 b. The operation of the digitalvalve packs is controlled by the control system 3.

FIG. 3A shows a simple roll nip N between the rolls of a roll pair 2,whose vibrations are attenuated with the control arrangement 1, whichincludes a control system 3, two digital valve packs 7; 7 a, 7 b, and ahydraulic actuator 5, the pressure of the hydraulic fluid prevailing ondifferent sides of the cylinder relative to the piston head beingcontrolled with said digital valve packs.

FIG. 3B shows a vibration measured in the roll nip of the apparatus ofFIG. 3A, the vibration having a given amplitude A1 and frequency f.

FIG. 3C shows a counter-vibration having a phase opposite to that ofFIG. 3B and generated by opening and closing the valves in the digitalvalve pack, and having a frequency f and an amplitude A2.

FIG. 3D shows an attenuated vibration in the roll nip, the vibrationhaving a frequency f and an amplitude A3. The attenuated vibration isthe sum vibration of the vibrations of FIGS. 3B and 3C.

FIG. 3E illustrates a simple roll nip N between the rolls of a roll pair2, whose vibrations are attenuated with the control arrangement, whichincludes a control system (not shown in the figure), a digital valvepack 7; 72 and a hydraulic actuator 5. The arrangement also comprises ananalog valve 7; 71, which serves for controlling the nip pressureprevailing in the roll nip and also the opening and closing of the rollnip. This is hence a hybrid system, whose switch means 7 includes bothan analog and a digital switch means.

FIG. 4 illustrates the reel cylinder 22 and the reel core 21 of thereeler 9. The fiber web W is reeled around the reel core 21, and in thisconjunction, the reel core needs to be displaced as the thickness s ofthe fiber web increases on the reel core 21. However, meanwhile, a givennip pressure needs to be maintained in the roll nip N in order to ensureregular reeling of the fiber web around the reel core. Both the locationof the reel core relative to the reel cylinder and the nip pressure inthe roll nip between the reel core and the reel cylinder are adjusted bythe control arrangement 1, which includes a control system 3, a digitalvalve pack 7, measurement means 4 and a hydraulic means 5. The form ofcontrol signals 31 determine whether to change the location of the reelcore with the digital valve pack relative to the reel cylinder, or theforce F exerted by the reel core on the reel cylinder, i.e. the nippressure prevailing in the roll nip. The same control arrangement 1 alsoenables attenuation of vibrations in the roll nip.

FIG. 5A shows an apparatus 10 for coating a fiber web which isconventional per se, comprising a roll pair 2 of two rolls, spaced bythe roll nip N. The fiber web W runs obliquely from the top downward andthe coating agent is transferred from the rolls 2 onto the fiber web inthe roll nip N. Inversely, the coating agent is transferred onto thesurface of the rolls 2; 21, 22 at coating stations (applicationstations) 6.

FIG. 5B illustrates an arrangement 1 for controlling the fiber webcoating apparatus of FIG. 5A, comprising a control system 3, sensors 4,which measure the nip pressure (or the force exerted by the roll in theroll nip) and also the position of the roll in the roll nip. The controlarrangement of the figure illustrates not only the application of thecontrol system of the invention to a fiber web coating apparatus, butalso the processing of measurement signals 41 from the sensors 4 by thecontrol system 3 into control signals 31, which control the switch means7, which is a digital valve pack.

FIG. 6A illustrates a control arrangement 1 of the invention, in whichpressurizing means 5; 51 within the mantle of a multi-zone roll 23 andpressurizing means 5; 52 outside the roll at the roll ends arecontrolled in accordance with the invention by digital valve packs 7; 71and 7; 72 and also the control system 3.

FIG. 6B shows a multinip calender 20, which comprises three idle rolls24 and a lower roll 23; 23 b and an upper roll 23; 23A, of which thelatter have design and operation of the pressurizing means inside andoutside the rolls identical to those of FIG. 6A. The figure illustratesthe implementation of the control arrangement 1 of the invention inmultinip calendars 20. Hydraulic actuators 5 connected both to the idlerolls and to the lower and upper rolls are controlled by means ofdigital valve packs 7, which, in turn receive their control signals 31from the control system 3.

FIG. 7A shows the control arrangement 1 of the invention on a blockdiagram level. The arrangement serves for measuring and controlling thenip pressure of the roll nip N and/or the location of the rolls or anyvariables acting on these.

FIG. 7B, in turn, shows a method of the invention on a block diagramlevel. The method measures and controls by means of the differencevariable the nip pressure of the roll nip N and/or the location of therolls, or any variables acting on these.

The invention is described in greater detail below.

The control of the nip pressure in the roll nip N between a roll pair 2of two rolls 21, 22 is illustrated in FIG. 1. The roll pair 2 may belocated in a calender, for instance, where a fiber web W runs betweenthe rolls, the fiber web being calendered (profiled) on its surface asit passes through the roll nip N. A hydraulic cylinder 5 is connected tothe roll 21 over a lever arm 52 (piston). The pressure of the hydraulicfluid of the compression cylinder 51 of the hydraulic actuator 5(hydraulic cylinder) is controlled by the digital valve pack 7. Thepressure of the hydraulic fluid in the compression cylinder generates aspecific force, by which the piston 52 acts on the roll 21. The roll 21then exerts a force F on the stationary backing roll 22, generating aspecific nip pressure in the roll nip N between the pair of rolls 2.

The pressure of the hydraulic fluid in the compression cylinder 51 isgenerated by opening one or more appropriate valves V; V1 to V8 of thedigital valve pack 7. The digital valve pack comprises eight valves V1to V8 of different sizes, the liquid flow passing through the valvesbeing doubled each time it passes from a smaller digital valve to thenext size. The difference between the volume flows of two digital valveswith consecutive volume flow rates is thus 100%, in other words, thevolume flow of a valve with greater volume flow is always double of thatof a valve with smaller volume flow. The valve sizes are then e.g. valveV1 one l/min., valve V2 two l/min, valve V3 four l/min., etc. When it isdesirable to generate e.g. a 10 kN nip pressure in the roll nip, thecontrol system 3 opens valve V1 in the digital valve pack 7, so thathydraulic fluid flows into the compression cylinder 52 at a rate of1l/min and the force F exerted by the roll 21 on the backing roll 22increases. Unless the force F or the nip pressure is desired, the valveV1 is closed and the valve V2 is opened, and the nip pressure and/or theforce F are monitored anew. This way of opening and closing the valvesV; V1 to V8 of the digital valve pack 7 aims at a valve combination thatbest realizes the desired nip pressure. The digital valve pack in FIG. 1comprises 8 valves, so that there are 2⁸=number of potential differentvolume flows, i.e. the digital valve pack has a resolution of 256. Whenthe principally adopted nip pressures are known, all the practicallyoccurring nip pressures can be realized by appropriately staggeredvolume flow rates of the individual valves in a digital valve pack andby an appropriate number of valves. Having but two states, the valvesincluded in a single digital valve pack have very rapid functions; eachvalve is either open or closed. With an open valve, the valve transmitsthe entire volume flow rate of hydraulic fluid allowed by the valve, andwith a closed valve, it is permeated by a zero amount of volume flow.Thus each digital valve operates on the on/off principle known indigital technology. The digital valve pack receives digital controlsignals from the control system 3. The control system, in turn, receivesthe pressure/force data it needs from the force sensor 4, which isconnected to the shaft 21 a of the roll 21.

The arrangement 1 opening and closing the roll nip N in FIG. 2 uses twodigital valve packs 7; 7 a; 7 b, which both comprise eight on/offvalves. By means of the valves in the digital valve pack 7 a, thepressure of the hydraulic fluid in the compression cylinder 5; 52 isincreased in the cylinder portion 51 a on the left-hand side of thepiston head 52 a of the piston 5; 51, and then the roll nip N opens. Bycontrast, by means of the valves in the digital valve pack 7 b, thepressure of the hydraulic fluid is increased in the cylinder portion 51b on the right-hand side of the piston head, so that the roll nipcloses. The rate of opening and closing the roll nip N, in turn, dependson the total volume flow rate of the opened valves. Opening differentvalve combinations achieves different opening rates of the roll nip,which depend on the cross-sectional area of the cylinder and on thefluid amounts flowing through the valves over a given period.

The roll nip is rapidly opened by opening all the valves in the digitalvalve pack 7; 7 b simultaneously, and then no separate rapid openingvalve will be necessary. Both the digital valve packs receive theirdigital control signals 31 from the control system 3. The controlsystem, again, receives the positional data 41 about the roll that itneeds from a sensor 4 measuring the roll location or position, thesensor being preferably located in the rear portion of the hydraulicactuator 5 with the hydraulic actuator viewed perpendicularly from thedirection of the roll nip. The roll location can be measured eitherrelatively to the backing roll or absolutely. Roll velocity data canalso be included in the measurement data, and the velocity data can bemeasured by means of an acceleration sensor, for instance.

The arrangement of the invention also allows for attenuation ofvibrations occurring in the roll nip in several devices used in paperand board machines, such as calenders, reelers, coating devices etc.FIGS. 3A-3E illustrate how to attenuate vibrations in a roll nip Nbetween the rolls 21, 22 in a roll pair 2 by means of the arrangement 1of the invention. Vibrations in the roll nip are often due tofluctuating control, hydraulic actuators, eccentric rolls, etc. The rollnip N of a calender is schematically shown in FIGS. 3A and 3E without afiber web passing through the roll nip, calender frame structures, etc.The control arrangement 1 in FIG. 3A includes a sensor 4 for measuringvibrations exerted on the frame of the backing roll 22, a control system3, two digital valve packs 7; 7 a, 7 b, and a hydraulic actuator(hydraulic cylinder) 5. Both the digital valve packs comprise eighton/off valves, so that both have a resolution covering 256 differentstates (volume flows). The valves in the digital valve packs 7 open andclose liquid flows in compression cylinder portions 51; 51 a, 51 blocated on different sides of the piston head 52; 52 a, and then thedigital valve packs can be used for increasing and decreasing the fluidpressure in the roll nip N. The opening and closing of the valves in thedigital valve packs 7 are controlled by the control system 3, whichreceives vibration data 41 from the sensor 4.

FIG. 3B shows a vibration occurring in the roll nip of the apparatus ofFIG. 3A, the vibration having been measured by the vibration sensor 4 ofthe arrangement 1 of FIG. 3A. This vibration has an amplitude A1 and afrequency f in the roll nip N. The vibration data 41 are transferredfrom the sensor 4 to the control system 3. The control system 3determines a counter-vibration (difference value) for the vibrationoccurring in the roll nip 3, the phase of this vibration differing fromthat of the vibration in the roll nip. The counter-vibration isdetermined on the basis of the amplitude of the maximum permissiblevibration (set value), for instance. After this, the control systemcontrols appropriate valves in the digital valve packs 7 in FIG. 3B sothat this particular counter-vibration realizes. The counter-vibrationis illustrated in FIG. 3C and it has a frequency f and an amplitude A2.Then the sum vibration in the roll nip is the sum of the vibrationsshown in FIGS. 3B and 3C, as shown in FIG. 3D. The amplitude of the sumvibration is A3 and its frequency is f. The amplitude A3 is smaller thanthe frequency A, implying attenuation of the vibration. This sumvibration can be remeasured by the sensor 4, and a suitablecounter-vibration can be determined for it under the control proceduredescribed above.

FIG. 3E, in turn, illustrates a control arrangement 1, in which theswitch means 7 comprises a digital valve pack 72 and a conventionalproportional valve 71. The nip pressure prevailing in the roll nip N isadjusted in a conventional manner per se by means of proportional valve71, which controls the pressure of the hydraulic fluid prevailing indifferent portions 51 a and 51 b of the compression cylinder 5; 51 bythe intermediation of fluid transfusion lines s1 and s2. The portion 51a of the compression cylinder is located on the left side of the pistonhead 52 a of the piston 52 moving in the compression cylinder 51, andaccordingly, the portion 51 b of the compression cylinder is located onthe right side of said piston head 52 a. With the analog control valve71 in position a, the hydraulic fluid flow follows the line s2 to theright side 51 b of the cylinder 51, while hydraulic fluid is dischargedfrom the left side 51 a of the cylinder along line s1. This increasesthe nip pressure in the roll nip N. On the other hand, with the controlvalve 71 in position b, the hydraulic fluid flow decreases the nippressure in the roll nip N, because the hydraulic fluid flow follows theline s1 to the portion 51 a of the compression cylinder 51, to the leftside of the piston head, and escapes along line s2 from the portion 51 bof the compression cylinder on the right side of the piston head. Shouldthe sensor 4 detect vibrations in the roll nip, they can be attenuatedby means of the on/off valves in the digital valve pack 72 by openingand closing digital valves as shown in FIGS. 3B to 3D, by acounter-vibration in a phase opposite to that of the measured vibration.The vibrations to be attenuated may also originate from the operation ofan analog control valve.

The control arrangement of FIG. 4 is used for positioning the reel core21 of a reeler 9 relative to the reel cylinder 22 and also forcontrolling the nip pressure of the roll nip N of a roll pair 2 formedof a reel cylinder and a reel core.

Should a conventional control arrangement with a large-sized controlvalve be used for shifting the reel core 21 of the reeler relative tothe reel cylinder 22 and for maintaining the nip pressure, the controlwould have a tendency to fluctuate: the change of volume flow ofhydraulic fluid required for maintaining the nip pressure between therolls 21, 22 is relatively small, whereas the change of volume flow ofhydraulic fluid required for shifting the location of the reel core insaid hydraulic means is relatively great. With the control switchingfrom positioning of the mutual location of the rolls 21, 22 to controlof the nip pressure prevailing in the roll nip between said rolls, orvice versa, the mass of a large-sized control valve is one reason ofproblems in passing from one control state to another, resulting in atendency of fluctuating control. Fluctuating control, in turn, causesirregular reeling of the fiber web onto the reel core.

In accordance with the invention, the on/off digital valves V includedin the digital pack are small-sized and have rapid operation. Thecontrol arrangement 1 illustrated in FIG. 4 comprises a digital valvepack 7, by means of which not only the position of the reel core 21 isadjusted relative to the stationary reel cylinder 22 but also the nippressure of the roll nip N between the reel core 21 and the reelcylinder 22. The control arrangement 1 comprises a control system 3,which receives data indicating the location of the reel core 21 from theposition sensor 4; 4 a and also receives continually or intermittentlyfrom the force sensor 4; 4 b measurement data 4 indicating the nippressure in the roll nip N or the force exerted by the reel core 21 onthe reel cylinder 22. The position sensor 4; 4 a detects the thickness sof the fiber web layer W on the reel core 21, the sensor being usuallylocated in the immediate vicinity of the outer surface of the fiber webW wound around the reel core.

Detection of the thickness of the fiber web layer can be performedeither by a mechanical position sensor as in the figure, or on the basisof any characteristic of the fiber web. In mechanical detection, theposition detector 4; 4 a is moved in the direction of the arrow with afull head, the thickness s of the fiber web layer increasing as theposition sensor sends the control system data about the position of theouter surface of the fiber web. In FIG. 4, the position sensor 4; 4 a isplaced on the side of the reel core, on top of the fiber web, and it ismoved in the direction of the arrow with a full head as the thickness ofthe fiber web layer increases. However, the position sensor could aswell be located at the end of the rear roll, and then the thickness ofthe fiber web layer would be measured by means of say, a photoelectricsensor. In some cases, the sensor may also measure a physical propertyof the fiber web, such as light transmission, for instance, which allowscalculation of the thickness s of the fiber web layer on the reel corein the control system 3. The arrangement also includes a force sensor 4;4 b for measuring the force F exerted by the reel core 21 on the reelcylinder 22. The force sensor operates only when the roll nip N isclosed. The force sensor can also be replaced with a pressure sensor,which measures directly the nip pressure prevailing in the roll nip Nbetween the reel cylinder and the reel core.

The analog signals 41; 41 a, 41 b measuring the position and force aretransferred from the force sensor 4; 4 b and position sensor 4; 4 a tothe control system 3, where they are processed under the controlfunction G(s) of the control system in order to control the pressure inthe roll nip and the position of the reel core 21 and the reel cylinder22 by means of the control signals 31 to be transmitted to the digitalpack 7. The control signals 31 sent from the control system 3 arealready in a digital form, so that they need not be converted intoanalog form, unlike control signals sent to analog valves. With the rollnip N closed, the pressure prevailing in the roll nip is adjusted on thebasis of measurement results 41; 41 b sent by the force sensor 4; 4 b byopening and closing appropriate valves in the digital pack by means ofcontrol signals 31. When the thickness s of the fiber web W around thereel core has increased to such an extent that the reel core 21 needs tobe displaced relative to the reel cylinder 22, appropriate on/off valvesV; V1 to V5 in the control pack 7 are opened so that the volume flow ofthe fluid entering the hydraulic cylinder 5 is sufficient for generatinga given hydraulic fluid pressure in the compression cylinder, which, inturn, generates the desired movement of the lever arm 5; 52 (piston)connected to the reel core 21. By altering the magnitude of the volumeflow, the velocity of movement of the reel core can be controlled in thedirection of the arrow with a full head. Even though the control modewere rapidly switched from control of the pressure in the roll nip N tocontrol of the mutual position of the reel core 21 and the reel cylinderand vice versa, there would be no notable control fluctuation, becausechanges in the volume flow are controlled by rapidly operating on/offvalves. In the arrangement 1 in FIG. 4, the digital valve pack 7 hasfive on/off digital valves V; V1 to V5, the control resolution of thisparticular digital valve pack comprising 2⁵=32 states, which is enoughfor most reelers. By increasing the number of valves contained in thedigital pack, even high resolutions are rapidly achieved; e.g. 16 on/offvalves already achieves a control resolution of 2¹⁶=65536 differentstates.

The control arrangement shown in FIG. 4 can also be connected withattenuation of vibrations generated in the roll nip. The amplitude andthe frequency of the vibrations are measured with acceleration or forcesensors, on the shaft of either of the rolls (21 or 22), for instance.The vibration signals are transferred to the control system 3, whichcontrols the valves of the digital valve pack 7 under its controlfunction G(s) to be switched open and off, so that the reel core 21 ismade to vibrate in a phase opposite to an artificially detectedvibration. The attenuation of the vibration is illustrated more indetail above in connection with FIG. 3.

FIGS. 5A and 5B illustrate the implementation of the arrangement of theinvention at a coating station and the conversion of measurement signalsfrom the sensors into control signals.

In FIG. 5A, the fiber web W passes through the roll nip N between theroll pair 2 formed of the roll 21 and the backing roll 22, the coatingagent being transferred onto the surface of the fiber web in the rollnip from the surface of the roll and its backing roll. The coating agentis transferred onto the surface s of the rolls 21, 22 from the coatingagent application stations 6; 61, 62, whose structure and operation areconventional per se. When it is desirable to open or close the roll nipN, the roll 21 is shifted relative to the backing roll 22 with ahydraulic cylinder 5 connected to a bearing housing of the roll 21 andsimultaneously the force exerted by the roll on the backing roll ischanged with the hydraulic cylinder while the roll nip N is closed. Whenthe fluid pressure of the cylinder portion below the piston head movingin the cylinder is increased, the roll nip opens, or when the roll nipis closed, the nip pressure decreases, whereas, as the fluid pressure ofthe cylinder portion above the piston head moving in the hydrauliccylinder is increased, the roll nip is closed, and with the roll nipclosed, the nip pressure increases. The position of the roll 21 relativeto the backing roll 22 is measured with position sensors (shown more indetail in FIG. 5B) located at the lower end of hydraulic cylinders ateach end of the roll 21 and detecting the position of the piston movingin the cylinder. The force exerted by the roll on the roll 21 in the nipN, in turn, is measured on the basis of the compression force betweenthe piston and the bearing housing, by means of a force sensor 4; 4 bconnected to the upper end of the piston. FIG. 5A shows a force sensor4; 4 b functionally connected to a hydraulic cylinder 5 located at thefirst end of the roll pair, i.e. the end illustrated in the figure, asimilar force sensor being provided at the other end of the roll pair 2.

FIG. 5B illustrates the processing of measurement signals 4; 41 arrivingfrom the force sensor 4; 4 b used in the apparatus of FIG. 5A and theposition sensor 4; 4 a and the control of the switch means 7 on thebasis of measurement signals. The force sensor 4; 4 b measurescontinuously the force exerted by the roll 21 on the backing roll 22 inthe roll nip N and indicates the force level as an analog measurementsignal 41; 41 b by means of the voltage (U). The position sensor 4; 4 a,in turn, measures continuously the position of the roll 21 relative tothe backing roll 22 and indicates the position as an analog measurementsignal 41; 41 a by means of the current level (A). The measurementsignals are transferred to a controller 3, which converts themeasurement signals 41; 41 a, 41 b into digital control signals 31 underits control function G(s). The control signals 31 are transmitted assuch to digital valve packs 7; 7 a, 7 b, which increase and decreaseliquid flows in portions of the cylinder 51 located on different sidesof the piston head 52; 52 a by means of on/off digital valves on thebasis of control signals 31. Digital valves of the digital valve pack 7;7 a serve to adjust the pressure of the hydraulic fluid in the cylinderportion 51 a on the left side of the piston head 52 a of the cylinder 51and digital valves of the digital pack 7; 7 b serve to adjust the fluidpressure of the cylinder portion 51 b on the right hand of the pistonhead 52 a.

The system may comprise a switch between the control system 3 and thedigital valve packs 7, for selecting the control mode between positioncontrol and force control, however, no such switch is usually needed,unlike a conventional control arrangement using both control valves andanalog connections, because the on/off valves contained in the digitalvalve pack have sufficiently rapid operation for switching the controlmode from position control to force control and inversely, almostwithout any delay. The control arrangement of the invention has theadditional marked advantage over an arrangement for controlling the rollposition and the roll nip pressure using analog control valves thatcontrol signals 31 from the controller 3 need not be converted intoanalog control signals, yielding simpler control of the arrangement andreduced loss of information during signal conversions.

FIG. 6A, again, is a simplified view of a “multi-zone roll” 23 equippedwith pressurizing means 5; 51 within the frame, and FIG. 6B shows theuse of such a “multi-zone roll” in a multinip calender 20. Themulti-zone roll has a stationary static frame 11 and hydraulic cylinders5, 51 connected to the frame, which can be pressurized in couples eachtime. A mantle 23 a rotates about the frame 11. Journaling 8 is providedbetween the mantle 23 a and the frame 11. The pressurizing of thehydraulic cylinders is controlled by the digital valve pack 7; 71, whichreceives control signals 31; 31 a from the control system 3. Hydrauliccylinders 5; 52 provided at the ends of the multi-zone roll serve tocontrol the calender pressure by the intermediation of the digital valvepack 7; 72. The digital valve pack 7; 72 controlling the calenderpressure is also connected to the control system 3, from which itreceives the control signals 31; 31 b.

Different parts of the mantle can be pressurized in different ways bymeans of hydraulic cylinders 5; 51 supported by the static roll frame11. The hydraulic cylinders 5; 51 are pressurized in couples each time,so that the illustrated multi-zone roll has five zones 51; 51 a, 51 b,51 c, 51 d, 51 e, each of which is pressurized with an individual fluidtransfusion duct. Each of said fluid transfusion ducts is connected toone of the on/off valves of the digital valve pack 7; 71, which arecontrolled by means of control signals 31 a from the control system 3.By opening and closing appropriate valves of the digital valve pack 7;71 the desired zones 51 under the mantle 23 a of the multi-zone roll canbe pressurized. At the ends of the multi-zone roll 23 shown in FIG. 6A,hydraulic cylinders 5; 52; 52 a, 52 b are provided, by means of whichthe multi-zone roll 23 can be raised and lowered. These hydrauliccylinders are controlled with a separate digital valve pack 7; 72, whichreceives control signals 31; 31 b from the control system 3. The numberof digital valves in the digital valve pack 7; 72 and the volume flowthey transmit are selected so that the desired pressure levels of thehydraulic fluid are generated in the hydraulic cylinders 5; 52 a, 52 b,as explained above in connection with FIG. 1.

The operation of the hydraulic cylinders 5; 45 at the ends of themulti-zone rolls of the type shown in FIG. 6A and that of thepressurizing means 5; 51 within the multi-zone rolls is controlled inconventional control arrangements by means of analog control valves andswitches. Such control arrangements are often susceptible to fluctuatingcontrol, because there are delays due to the operation of the controlvalves during the changes in the pressurization of different zones 51 ato 51 e. By contrast, in the control arrangement of the invention shownin FIG. 6A, the liquid flow from the hydraulic station (not shown in thefigure) to the pressurizing means 51 is controlled with the digitalvalve pack 7; 71, which has 5 on/off valves. Each valve opens and closesa fluid transfusion duct leading to a given hydraulic cylinder pair 51;51 a to 51 e under the roll mantle. The digital valves have rapidoperation, so that the pressurization in the different roll zones can bequite rapidly changed, allowing crown variation control requirementscaused by the weight of the roll to be rapidly met. The other digitalvalve pack 7; 72, in turn, serves to change the nip pressure of the rollnips and also to open and close roll nips by varying the fluid pressureof hydraulic cylinders 5; 52; 52 a, 52 b at the ends of the roll. Theroll nips can also be opened/closed at the desired rate byopening/closing appropriate valves of the digital pack, as explainedabove in conjunction with FIG. 2. A conventional prior art controlsystem for controlling the functions of a multinip calender comprises amicrocomputer, which receives continuously information about the nipparameters from measurement sensors measuring these parameters and whichtransmits, on the basis of these data, control signals to hydrauliccylinders controlling the crown variation within the rolls andpressurizing the mantle and to hydraulic cylinders adjusting the nippressure by means of analog valves and switches. Before the controlsignals are sent, they are converted from a digital form into an analogform with a view to controlling analog control valves. By contrast, inthe control arrangement of the invention, control signals 31 in digitalform coming from the control system 3 need not be converted into analogform, because the control valve(s) have been replaced with digital valvepacks, whose control signals are digital.

Multi-zone rolls are often used as the uppermost or lowermost rolls andalso as idle rolls in multinip calenders. FIG. 6B illustrates anexemplified vertically directed multinip calender 20, in whichmulti-zone rolls of the kind shown in FIG. 6A have been used as theuppermost roll 23; 23 a and the lowermost roll 23; 23 b in the set ofrolls. The multi-zone rolls 23; 23 a and 23 b comprise pressurizationmeans within the rolls as shown in FIG. 6A and hydraulic cylinders 5; 52a, 52 b have been connected to these rolls to be used for generating thedesired nip pressure distribution and nip pressure in the multinipcalender 20. In addition, these hydraulic cylinders serve to open andclose calendering nips N in the set of rolls during a path interruption,for instance. There are no loading means within the rolls among the idlerolls 24; 24 a, 24 b, 24 c between the uppermost roll 23; 23 a and thelowermost roll 23; 23 b, however, loading arms 12 have been connected totheir bearing housings, and in turn, hydraulic cylinders 53; 53 a, 53 b,53 c have been connected to the loading arms for compensating the weightof the masses of the auxiliary means at the ends of these idle rolls,such as steam boxes and removal rolls (not shown in the figure). Inaddition to multinip calenders such as supercalenders, multi-zonecalenders are generally used in presses for dewatering the fiber web.

The multinip calender 20 shown in FIG. 6B uses the control arrangement 1of the invention for controlling the nip loads and nip load profiles ofthe roll nips N of a set of rolls. The control system 3 receivescontinuously information about the nip parameters from measurementsensors (not illustrated) measuring these parameters and controls thehydraulic cylinders 53; 53 a, 53 b, 53 c compensating the weight of theauxiliary means on the basis of these data by sending control signals31; 31 d to the digital valve packs 7; 71. The number of on/off valvesof the digital valve packs and the flow rate ratios have been selectedsuch that the digital valve packs 7; 71 a, 71 b, 71 c allow optimalcompensation of the loads caused by the weight of the auxiliary means ofthe rolls 24; 24 a, 24 b, 24 c. Each of the digital valve packs 71; 71a, 71 b, 71 c shown in FIG. 6B has five on/off valves, so that each ofthem is able to control 2⁵=32 different load compensating states. Thecontrol system 3 also controls the calendering pressure of the set ofrolls and hydraulic cylinders 5; 52 a, 52 b functionally connected tothe uppermost and the lowermost roll and controlling the opening and theclosing of the roll nips by sending control signals 31; 31 c to thedigital valve pack 7; 72. The digital valve packs 7; 72; 72 a, 72 bcontrolling the calendering pressure and the opening and closing of thenips may be identical or different. Each of the digital valve packs 72shown in FIG. 6B has five on/off valves, so that they can achieve 2⁵=32different control states of calender loading and for the rate ofopening/closing the roll nips N.

FIGS. 7A and 7B illustrate the control arrangement and method of theinvention as a block diagram.

FIG. 7A is a block diagram of the control arrangement 1 of the inventionfor controlling the position and/or force of two elongated rollingdevices in the roll nip N between two elongated rolling device pairs 2in a paper machine. The rolling devices comprise a roll and its backingroll or a roll and a doctor blade, for instance. The control arrangement1 comprises, as shown in FIG. 7A, a measuring means 4 for measuring theposition and/or force of the rolling device or any variable acting onthese and for sending a measurement signal 41 to the control system 3.The control arrangement 1 further comprises a hydraulic means 5, bymeans of which the position and/or force of the rolling device ischanged in the roll nip, a switch means 7 for controlling the volumeflow of the hydraulic means and a control system 3 for receiving ameasurement signal 41 and for comparing the information contained in themeasurement signal with the set value of the variable in order togenerate a control signal 31 and to transmit it to the switch means 7.The switch means has receive means for receiving and processing acontrol signal and at least one digital valve pack having on/off valves,which can be opened and closed on the basis of a control signal in orderto change the fluid pressure of the hydraulic means and/or the liquidflow rate to the hydraulic means.

In accordance with FIG. 7B the control method, in turn, serves tocontrol the position and/or force of an elongated rolling device in theroll nip N between the rolling device pair 2 formed of two elongatedrolling devices in paper machines. The position of the rolling devicerelative to the other rolling device and/or the force exerted by therolling device on the other rolling device or any variable acting onthese is measured. The measured variable value is compared with the setvalue of said variable in order to obtain the difference value of thevariable. The difference value is used as a basis for adjusting theposition of the rolling device and/or the force it exerts on the otherrolling device with the aid of the hydraulic means. The fluid pressureof the hydraulic means and/or the liquid flow rate to the hydraulicmeans is changed in order to alter the difference value by openingand/or closing at least one on/off valve in the digital valve packconnected to the hydraulic means.

Only a number of embodiments of the method of the invention and of therelated control arrangement has been described above, and it is obviousto those skilled in the art that the invention can be implemented alsoin other ways within the scope of the inventive idea defined in theclaims.

Consequently, arrangements utilizing a digital valve pack can be appliedfor partly or completely compensating loads caused by nip pressures ofvertically positioned multinip calendars, opening and closing velocitiesof the roll nip and auxiliary equipment of the idle means, which are ofthe type disclosed by DE patent specification 10101182.

With the use of the arrangement of the invention, the mass of rolls canalso be controlled in an arrangement of the kind disclosed by DE patentapplication 10006299, in which the valve 32 shown in FIG. 2 of thepatent application is replaced with a digital valve pack, which closesand opens rapidly flows from pumps 23 and 25, allowing the velocity ofmovement of the piston moving within the roll and the fluid amountswithin the roll to be rapidly changed.

In the embodiment illustrated in FIGS. 5A to 5B above, the nip pressureof the roll nip and the opening and closing of the roll nip are adjustedin the apparatus used for fiber web coating by means of the controlarrangement of the invention. The coating agent is transferred onto thefiber web in the roll nip between the roll and its backing roll from thesurface of the roll and/or the backing roll or from endless beltsrotating about the roll and/or its backing roll. The coating agent istransferred onto endless belts of the roll and/or its backing roll orrotating about the roll and/or its backing roll at application stations,which in several embodiments include an application means (=rollingdevice) pressed against the roll or the endless belt rotating about theroll, such as a blade or a rod. The load pressure between theapplication means and the roll or the endless roll rotating about theroll can be changed with a hydraulic actuator connected to theapplication means, such a hydraulic cylinder, in order to control thethickness and smoothness of the coating agent. The arrangement of theinvention allows rapid and precise action on the load pressure betweenthe application means and the roll or the endless belt rotating aboutthe roll by conducting the hydraulic fluid flow passing to the hydraulicactuator through the digital valve pack, which has an appropriate numberof on/off valves for achieving the desired load pressure level.

The load pressure between the doctor blade (=rolling device) and theroll surface can also be altered with the arrangement of the inventionin doctor blades wiping the roll surface, which are commonly used inapparatuses for calendaring a fiber web, among other things, byconducting the hydraulic fluid flow pressing the doctor blade againstthe roll through the digital valve pack, which has an appropriate numberof on/off valves for achieving the desired load pressure level.

The examples above describe the use of digital valves having two states.Digital valves may also have several states. Thus, a digital valvehaving say, three states could transmit oil into two directions, and inone position, it would not allow fluid to permeate at all. The operationof the valve can then be depicted as follows: State+1: the valvetransmits oil into a first direction, to the front side of the piston inthe cylinder, for instance.

State 0: the valve is closed and does not transmit fluid.

State−1: the valve transmits oil into a second direction, e.g. to therear side of the piston in the cylinder, i.e. to the side of the pistonrod.

Such a valve would operate in the way of an analog servo valve (thevalve being closed in the center of the spindle), but would open to 100%or by digital steps each time. This allows the same valve to drive thenip into closed position with a full flow or to drive it into openposition with a full flow, the opening/closing velocity of the roll nipdepending on the size of the valves/valve combinations of the digitalpack used in each case. The three-step valve digital valve mentionedabove (having three states) is also preferably used in the vibrationcontrol of the roll nip, and then the digital valve can transmit oilinto two directions.

1. A method for controlling position or force in an apparatus which hasa roll nip between a first elongated rolling device and a secondelongated rolling device in a paper or a board machine, the methodcomprising the steps of: measuring a variable related to a position ofthe first elongated rolling device relative to the second elongatedrolling device or the force exerted by the first elongated rollingdevice on the second elongated rolling device; comparing the value ofthe measured variable to a preset value of said variable in order toobtain a difference value of the variable; adjusting on the basis of thedifference value of the variable, the position of the first elongatedrolling device with respect to the second elongated rolling device orthe force the first elongated rolling device exerts on the secondelongated rolling device; and changing a fluid pressure of a hydraulicdevice or changing a rate of flow of a fluid to the hydraulic device inorder to alter the difference value of the variable, by repeatedlydriving with a digital signal a plurality of digital valves arranged inparallel to form a flow of fluid which is the sum of the flow from eachof said plurality of digital valves, wherein each valve of the pluralityhas a finite number of discrete states, and wherein driving theplurality of digital valves comprises changing the plurality of digitalvalves from a first condition where all of the plurality of digitalvalves are in first particular discrete states directly to a secondcondition, different from the first condition, wherein at least one ofthe plurality of digital valves assumes a different discrete state, andwherein the hydraulic device is connected to the second elongatedrolling device and changes the value of the measured variable.
 2. Themethod of claim 1 wherein the digital valves of the plurality of digitalvalves have only two states, open and closed, and wherein the differencevalue is obtained digitally and defines a digital difference value andwherein the step of changing a fluid pressure of a hydraulic device orchanging a rate of flow of a fluid to the hydraulic device comprises thestep of, on the basis of the digital difference value, opening selecteddigital valves of the plurality of digital valves whose flow volumeachieves a decrease of the difference value.
 3. The method of claim 1wherein the digital valves of the plurality of digital valves have onlytwo states, open and closed, and wherein the step of measuring avariable comprises measuring the position of the first elongated rollingdevice in the roll nip relative to the second elongated rolling device;and wherein the step of changing a fluid pressure of a hydraulic deviceor changing a rate of flow of a fluid to the hydraulic device comprisesthe step of opening selected digital valves of the plurality of digitalvalves, whose flow volume achieves a decrease of the difference value ata selected rate.
 4. The method of claim 1 wherein the digital valves ofthe plurality of digital valves have only two states, open and closed,and wherein the step of measuring a variable comprises measuring anamplitude and frequency of vibration in the nip formed between the firstelongated rolling device and the second elongated rolling device, andfurther comprising: generating a control signal which is an inverse ofthe measured amplitude and frequency of vibration of the device; whereinthe step of adjusting on the basis of the difference value is anadjustment based on the control signal; wherein the step of changing afluid pressure of the hydraulic device or changing a rate of flow of afluid to the hydraulic device comprises using the control signal tochange the rate of flow of the fluid to the hydraulic device by openingand closing selected digital valves of the plurality of digital valveson the basis of the control signal in a phase opposite to the vibrationso as to actively attenuate the vibration.
 5. An arrangement forcontrolling position or force of an elongated rolling device in a rollnip between a first elongated rolling device and a second elongatedrolling device, in a paper or board machine, the arrangement comprising:a measuring device arranged to measure at least one variable related toposition or force of the first elongated rolling device to produce ameasurement signal; a control system in measurement receiving relationto the measuring device, the control system arranged to compare themeasurement signal with a selected set value of the variable to generatea control signal; a hydraulic device arranged to change the position orforce of the rolling device in the roll nip with a fluid pressure or aflow rate of the fluid; and a switch connected in control signalreceiving relation to the control system, the switch having at least onefirst plurality of digital valves connected in parallel so as to providea sum volume flow of fluid which is the sum of the flow from each ofsaid plurality of digital valves, wherein each valve of the pluralityhas only two or three discrete states wherein each discrete state ofeach valve is either open or closed and is switchable between saiddiscrete states and wherein the hydraulic device is connected in sumflow volume receiving relation to at least one first plurality ofdigital valves so that the fluid pressure in the hydraulic device or theflow rate of the fluid to the hydraulic device can be changed byregulating the sum volume flow of fluid to the hydraulic device.
 6. Thearrangement of claim 5, wherein each valve of the plurality of digitalvalves is of a size different from the other valves of the plurality ofdigital valves, and wherein the size of each valve of the plurality ofdigital valves is selected to form a series of progressively largerdigital valves starting with a first digital valve, and wherein eachprogressively larger valve has twice the flow capacity of the precedingvalve in the series.
 7. The arrangement of claim 6, wherein themeasuring device is arranged to produce an analog measurement signal andwherein the control system includes an A/D converter having a digitaloutput connected to the switch.
 8. The arrangement of claim 7, whereinthe digital output does not pass through a D/A converter.
 9. Thearrangement of claim 5, wherein the switch in addition to having the atleast one first plurality of digital valves has an analog valve arrangedin parallel with the first plurality of digital valves, arranged tosupply the majority of the flow rate of the fluid to control theposition of the first elongated rolling device or of the force the firstelongated rolling device exerts on the second elongated rolling devicein the roll nip.
 10. The arrangement of claim 5, wherein the switchfurther comprises a second plurality of parallel connected digitalvalves connected to form a second volume sum flow of fluid which is thesum of the volume flow from each valve of said second plurality ofdigital valves, wherein each valve has only two discrete states, openand closed, and is switchable therebetween, and wherein the hydraulicdevice is connected in sum flow volume receiving relation to the secondplurality of digital valves the hydraulic device being arranged to openand close the roll nip between the first elongated rolling device andthe second elongated rolling device.
 11. The arrangement of claim 10,wherein the hydraulic device is a hydraulic cylinder having a pistonhead having a first side and a second side, and a first cylinder portionlocated on the first side of the piston head is connected in sum flowvolume receiving relation to at least one first plurality of digitalvalves, and a second cylinder portion located on the second side of thepiston head is connected in sum flow volume receiving relation to thesecond plurality of digital valves.
 12. The arrangement of claim 11,wherein the roll nip is arranged to be rapidly opened by opening all ofthe first plurality of digital valves.
 13. The arrangement of claim 5,wherein the first elongated rolling device is a reel core, about which afiber web is reeled, and wherein the second elongated rolling device isa reel cylinder, having a surface arranged to receive the fiber web andfeed the fiber web into the roll nip which is located between the reelcore and the reel cylinder; wherein the hydraulic device is arranged tochange the nip pressure in the roll nip by being functionally connectedto the reel core, said hydraulic device additionally arranged to shiftthe position of the reel core relative to the reel cylinder; and whereinthe measuring device is arranged to measure the force exerted by thereel core on the reel cylinder in the roll nip or is arranged to measurethe position of the reel core relative to the reel cylinder.
 14. Thearrangement of claim 13, wherein: the measuring device is arranged todetect amplitude and frequency of the reel core position which defines avibration occurring in the reel core; and wherein the control system isarranged to determine a counter-vibration and to generate acounter-vibration control signal; and wherein the switch is connected incontrol signal receiving relation to the control system and is arrangedto control vibration by regulating the volume flow of fluid to thehydraulic device.
 15. The arrangement of claim 5, wherein the firstelongated rolling device and the second elongated rolling device arecoating rolls, and are arranged to apply coating agent or coating pasteonto one or both sides of a fiber web passing through the roll nip. 16.The arrangement of claim 15, further comprising an application means,with the aid of which the coating agent or coating paste is applied to asurface of a first coating roll or of an endless belt rotating about thecoating rolls.
 17. The arrangement of claim 5 wherein first elongatedrolling device and the second elongated rolling device are rolls in amulti-nip calender and load reduction means are provided at least at theend of one said rolls; wherein the hydraulic device is a hydraulicactuator provided at the end of one of said rolls; and wherein the atleast one first plurality of digital valves is arranged for controllingthe hydraulic actuator so that the hydraulic actuator compensates forloads caused by auxiliary equipment on the one of said rolls.
 18. Thearrangement of claim 17, wherein an additional plurality of digitalvalves is connected in parallel to form an additional sum flow volume offluid which is the sum of the volume flow from each valve of saidadditional plurality of digital valves, wherein each valve of theadditional plurality of digital valves has only two discrete states:open and closed, and is switchable therebetween, and wherein theadditional plurality of digital valves are connected in sum flow volumetransmitting relation to control hydraulic actuators within the one ofsaid rolls for pressurizing different zones of a roll mantle of the oneof said rolls.
 19. The arrangement of claim 17, wherein the hydraulicactuator provided at the end of the one of said rolls is arranged toopen and close the roll nip.
 20. The arrangement of claim 5, wherein thefirst and the second elongated rolling device are rolls having loadingdevices therewithin, and wherein said loading devices are arranged to becontrolled with the at least one first plurality of digital valves. 21.The arrangement of claim 5, wherein the first elongated rolling deviceis a doctor blade and wherein the hydraulic device is a hydraulicactuator arranged to control the nip pressure of the roll nip betweenthe doctor blade and the second elongated rolling devices.
 22. Themethod of claim 1 wherein the step of driving the plurality of digitalvalves comprises driving at least 5 valves wherein each valve of the atleast 5 valves has two discrete states: open and closed, and wherein thefirst condition and the second condition are selected from at least 32possible different conditions the at least 5 valves can be in.
 23. Themethod of claim 1 wherein the step of driving the plurality of digitalvalves comprises driving at least 8 valves wherein each valve of the atleast 8 valves has two discrete states: open and closed, and wherein thefirst condition and the second condition are selected from at least 256possible different conditions the at least 8 valves can be in.
 24. Themethod of claim 1 wherein the step of driving the plurality of digitalvalves comprises driving at least 12 valves wherein each valve of the atleast 12 valves has two discrete states: open and closed, and whereinthe first condition and the second condition are selected from at least4096 possible different conditions the at least 12 valves can be in. 25.The method of claim 1 wherein the step of driving the plurality ofdigital valves comprises driving at least 16 valves wherein each valveof the at least 16 valves has two discrete states: open and closed, andwherein the first condition and the second condition are selected fromat least 65,536 possible different conditions the at least 16 valves canbe in.
 26. The arrangement of claim 5 wherein the at least one firstplurality of digital valves comprises at least 5 valves wherein eachvalve of the at least 5 valves has two discrete states, open and closed,so that the at least one first plurality of digital valves have incombination at least 32 possible different conditions.
 27. Thearrangement of claim 5 wherein the at least one first plurality ofdigital valves comprises at least 8 valves wherein each valve of the atleast 8 valves has two discrete states, open and closed, so that the atleast one first plurality of digital valves have in combination at least256 possible different conditions.
 28. The arrangement of claim 5wherein the at least one first plurality of digital valves comprises atleast 12 valves wherein each valve of the at least 12 valves has twodiscrete states, open and closed, so that the at least one firstplurality of digital valves have in combination at least 4,096 possibledifferent conditions.
 29. The arrangement of claim 5 wherein the atleast one first plurality of digital valves comprises at least 16 valveswherein each valve of the at least 16 valves has two discrete states,open and closed, so that the at least one first plurality of digitalvalves have in combination at least 65,536 possible differentconditions.